Original General Order 95
(f) Determination of Amount of Sag for Various Points in a Span
The sag curves on pages 271 to 276 (charts
, inclusive, show for wires of different sizes and materials the value of
the center sag at which these wires should be strung under normal conditions
to have the assumed factors of safety under the designated load conditions.
At times it is desirable to know, not only the amount of sag at the center
of the span, but also the amount of sag at some other point in the span.
This is necessary, for example, in obtaining the clearance over
other wires where the point of crossing between the crossing span and the
wires crossed occurs, not at the center of the crossing span, but at some
On page 279, (chart 9)
a curve is given by means of which, given the amount of center sag, the
amount of the sag at any other point in the span can be determined. This curve
gives the value of the sag at all points on the catenary curve expressed in
per cent of the center sag. The use of this curve is shown by the following
Assume - A span of 300 feet-Heavy Loading District.
A center sag, determined from the sag curves, of 5.30 feet.
The crossing span crosses over a Class C line, on which the top
wire at the point of this crossing has an elevation of 25 feet.
This point of crossing to be 105 feet from the
nearest support of the crossing conductor, and a minimum vertical clearance
of 6 feet is required at the point of crossing.
- At what height must the crossing
conductor be supported in order that this required vertical clearance shall
As the span length is 300 feet, and
the distance from the nearest support to the point of crossing is 105 feet,
this distance is 35% of the span length. From the curve, page 279, (chart 9)
, the value of the sag at this point is 91 per cent of the center sag. The
sag at this point, therefore, equals 5.30 x 0.91 = 4.82 feet.
Therefore, the required elevation
of the crossing conductor at its point of support is equal to the height of
the Class C wires crossed (25 feet), plus the minimum vertical clearance required
(6 feet), plus the sag of the conductor at the point of crossing (4.82 feet),
25 feet + 6 feet + 4.82
feet = 35.82 feet